When you share your calendar using Microsoft Office Outlook 2007, you may receive this error message:
Error while preparing to send sharing message.
Also, if you try to check the permissions on the calendar, you may receive this error message:
An error occurred when setting schedule permissions.
This is because of a limitation between Exchange Server 2013 and Exchange Online
If you experience this issue and want to share the calendar or to set up calendar permissions, please use the Outlook Web App.
For information on how to do this, see the Microsoft support article.